In one type of EAS system, the marker includes both a high-permeability low-coercive force portion, and at least one magnetizable section having a higher coercive force than the low-coercive force portion. When the higher coercive force section is magnetized, it alters the detectable signal otherwise produced. Such markers are known as "dual status" markers. An example of a dual status marker is taught in U.S. Pat. No. 4,825,197 (Church and Heltemes).
EAS systems of this type are, for example, disclosed and claimed in U.S. Pat. No. 3,665,449 (Elder and Wright). As they set forth at column 5, lines 10 to 39, a dual status marker of the type described above may be "sensitized"(i.e., the higher coercive force section demagnetized) by placing the marker in a large AC field, and gradually withdrawing the marker.
German Offenlegungsschrift DE 30 14 667 Al (Reiter) depicts a type of desensitizer employing a resistive-inductive-capacitive (RLC) circuit to produce magnetic fields which steadily alternate in polarity and decrease in magnitude. The magnetic fields are produced by winding the inductive coils around rib-like cores arranged about the desensitization region. The directions of the windings around the coils alternate, and thus the polarities of the magnetic fields produced alternate. Thus, when the circuit is activated, sharply defined magnetic zones of alternating polarity arise, through which the article affixed with a marker may be passed.
While such techniques may be useful for the markers affixed to a wide variety of articles, the magnetic fields required for effective resensitization interfere with magnetic states associated with certain articles. For example, the compact size and popularity of prerecorded magnetic audio and video cassettes make such articles frequent targets for shoplifters, and hence likely articles on which EAS markers would be affixed. However, in a rental situation, when such markers are resensitized upon return from rental, a resensitizer apparatus as described above may unacceptably affect the signals prerecorded on the magnetic tapes within the cassettes. Similarly, magnetic disks (flexible or otherwise) or any other magnetic data storage medium may be affected by the resensitizer apparatus.
Commercial embodiments of resensitizers are the Model 950 and 951 resensitizers available from the Minnesota Mining and Manufacturing Company (3M). Another embodiment is taught in U.S. Pat. No. 4,752,758 (Heltemes).